1. Field to which invention relates
The invention relates to the production of ceramic mouldings and is more particularly concerned with the production of magnetized ferrite mouldings.
2. The prior art
In the case of a prior art method for the production of magnetized ferrite mouldings the ferrite compositions were ground in suitable mills to particle sizes of approximately 1 micron. During the course of grinding water was added so that an aqueous suspension of the particles was produced. This aqueous suspension was then filled into the dies of presses and the ferrite particles were aligned by magnetic fields. The water was pressed out by the filter provided in the presses and as a result the orientated particles were mechanically fixed in the green pressing. After final drying of the green pressings, still containing approximately 6 to 10% of water, the pressings were sintered at temperatures of approximately 1200.degree. C and after cooling were ground down to the finished dimensions.
Since the preliminary filtering out of water during the pressing operation requires a substantial amount of time and furthermore the presses and tools represent expensive and complicated precision machines, there has always been the desire to simplify the presses and also the pressing operation in order to be able to produce such mouldings in a more economic manner. One method of achieving this is by the production of such pressings by so-called, dry pressing methods. In the case of such methods, the desired increase in production speed can be produced, however, the magnetic parameters of the resulting product are substantially worse than in the case of mouldings which are produced in accordance with the wet pressing method.
Since in recent times high requirements have arisen as regards the magnetic quality of such mouldings, it has been necessary as a matter of necessity to revert to the wet pressing method, which is accompanied by the long filtering times already mentioned as being disadvantageous.
In order to be able to achieve high pressing speeds while using the wet pressing method, the practice has already been adopted of filling the die from above with the upper punch raised, moving the upper punch down towards the die, and, by pressing down of the die using the upper punch against a stationary lower punch, pressing out the water through a filter incorporated in the upper punch. It is possible in this manner to mechanically fix the alignment of the particles in the green pressing with approximately 8% residual water.
However, this method as well also operates too slowly. Therefore, attempts have been magnetizable to improve the method by the introduction of multiple tools. The saving in working time by the simultaneous pressing of several mouldings, however, leads to substantial difficulties due to the sealing, which becomes necessary as a result, of several punches against the dies. The necessarily high pressing forces in the order of magnitude of 400 kp per sq. centimeter lead to substantial difficulties as regards the maintenance of the seals, since even if only one seal is defective, it automatically follows that all other pressing stations must be made inoperative during the repair.
Therefore, in the case of the manufacture of ceramic mouldings, more particularly of magnetized ferrite mouldings, these methods have been dropped and the practice has been adopted of filling a suspension of moulding composition particles under pressure into a filling space and removing the liquid via a filter formed by a chamber wall provided with filter openings. Since long filling ducts connect the individual pressing station with the closed chamber to allow direct pressure equalisation, individual sealing of the lower punches or rams is no longer necessary. Instead it is only necessary to provide a common or joint seal, which can always be made round, between the central column and the chamber.
In the case of this chamber filling method, however, certain filling times are still required and the removal of water is generally carried out after conclusion of filling and in the actual following pressing operation itself.
A further advance is obtained if the filling is carried out under increased pressure by means of cylinder metering pumps.
In the case of this method, in the production of magnetized ferrite mouldings the magnetic field can be switched on during the process itself and accordingly the ferrite particles can be orientated during the filling. The pressure present during filling makes possible moreover, even during filling itself, at least a partial removal of water and, therefore, a shortening of the pressing operation. It can readily be seen that in employing such a method, the filling devices, more particularly the pumps are subjected to a substantial load and therefore wear very quickly.